1. Field of the Invention
The present invention relates to an electrode for a fuel cell and a method of manufacturing the same. More specifically the present invention pertains to an electrode in a fuel cell that receives supplies of gaseous fuel and oxidizing gas and generates an electromotive force, wherein an electrochemical reaction proceeds with a predetermined component included in the supply of gas. The present invention also pertains to a method of manufacturing such an electrode.
2. Description of the Prior Art
Fuel cells convert the chemical energy of a fuel directly into electrical energy and are expected to attain the high energy efficiency. In a fuel cell, for example, in a polymer electrolyte fuel cell, a supply of gaseous fuel containing hydrogen and a supply of oxidizing gas containing oxygen are fed respectively to a pair of electrodes that are arranged across an electrolyte film, so that the following electrochemical reactions proceed: EQU H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- (1) EQU 2H.sup.+ +2e.sup.- +(1/2)O.sub.2 .fwdarw.H.sub.2 O (2) EQU H.sub.2 +(1/2)O.sub.2 .fwdarw.H.sub.2 O (3)
Equation (1) shows the reaction proceeding on the anode in the fuel cell, Equation (2) shows the reaction proceeding on the cathode in the fuel cell, and Equation (3) shows the overall reaction proceeding in the fuel cell. These reactions generally proceed in the regions called catalyst electrodes. The catalyst electrode is disposed between the electrolyte film and a gas diffusion layer, which diffuses the supply of gas fed to the electrolyte film. The catalyst electrode includes a catalyst for accelerating the above electrochemical reaction and an electrolyte. The supply of gas is fed to the catalyst electrode via the gas diffusion layer. The above electrochemical reaction proceeds on the catalyst of the catalyst electrode by utilizing a reactive substance (hydrogen or oxygen) included in the gas. For the continuous and smooth progress of the electrochemical reaction, it is required to sufficiently diffuse the gas in the catalyst electrode, supply a sufficient amount of the reactive substance included in the gas to the catalyst, and ensure the sufficient transmission pathway of electrons and protons, which contribute to the reaction, in the catalyst electrode.
In a known structure for ensuring the sufficient progress of the reaction in the catalyst electrode, the catalyst electrode is made of a film consisting of a catalyst past containing a catalyst, an electrolytic solution, and a predetermined solvent (for example, JAPANESE PATENT LAYING-OPEN GAZETTE No. 5-507583). In this prior art technique, the catalyst paste is produced by soaking carbon fine particles with a catalyst carried on the surface thereof in an electrolytic solution containing an electrolyte having the ion-conducting ability, and this catalyst paste is dried to form the catalyst electrode. This arrangement enables the surface of the catalyst-carrying carbon granules to be homogeneously coated with the electrolyte and allows the ions required for the electrochemical reaction proceeding on the catalyst and the ions produced through the electrochemical reaction to readily move in the vicinity of the catalyst, thereby ensuring the sufficient ion-conducting ability in the catalyst electrode.
FIG. 7 schematically illustrates the structure of a catalyst electrode manufactured by coating the surface of the catalyst-carrying carbon granules with the electrolyte. The drawing of FIG. 7 shows a catalyst electrode 123 functioning as the cathode as an example. In the catalyst electrode 123, the surface of each catalyst cluster, which is formed by aggregation of catalyst-carrying carbon granules 136 with a catalyst 134 carried thereon, is coated with an electrolyte layer 138. As shown in FIG. 7, in the catalyst electrode 123 functioning as the cathode, a supply of protons, which is fed from the anode and passes through the electrolyte film, is moved towards the gas diffusion layer via the electrolyte layer 138 that coats the surface of the catalyst 134. In this process, the protons are fed to the catalyst 134. A supply of electrons (not shown) is fed with oxygen molecules from the gas diffusion layer to the catalyst 134 on the surface of the catalyst-carrying carbon granules 136. The reaction of Equation (2) given above accordingly proceeds on the catalyst 134. The oxygen molecules penetrate the electrolyte layer 138 and reach the catalyst 134 on the surface of the catalyst-carrying carbon granules 136.
Another proposed method for manufacturing a catalyst electrode makes a catalyst carried on the surface of a gas diffusion electrode, applies a solution, in which solid acid particles having the ion exchange ability are dispersed, on the surface of the catalyst-carrying gas diffusion electrode, and joins the applied surface of the gas diffusion electrode with an electrolyte film, which constitutes the electrolyte layer of the fuel cell (for example, JAPANESE PATENT LAID-OPEN GAZETTE No. 6-36776). In this proposed method, formation of the layer having the ion exchange ability between the catalyst and the electrolyte film ensures the sufficient ionconducting ability in the catalyst electrode.
In the proposed structure that coats the surface of the catalyst with the electrolyte, the greater amount of the electrolyte to form the thicker electrolyte layer for coating the surface of the catalyst ensures the greater transmissible area for the ions moving in the catalyst electrode (in the case of FIG. 7, the protons moving from the electrolyte film towards the gas diffusion layer) and attains the better ion-conducting ability. The thicker electrolyte layer, however, makes it difficult for the reactant (oxygen in the case of FIG. 7) included in the gas to penetrate the electrolyte layer and reach the surface of the catalyst. Namely the thicker electrolyte layer interferes with the gas diffusion to the catalyst (that is, the supply of the reactant to the catalyst) and lowers the gas diffusion rate in the catalyst electrode. This results in lowering the contribution efficiency of the catalyst involved in the electrochemical reaction and worsening the performance of the catalyst electrode. The lowered gas diffusion rate due to the thicker electrolyte layer is prominent especially in the case of oxygen molecules, which are greater in size than hydrogen molecules. Namely the cathode is liable to be affected more significantly than the anode. The less amount of the electrolyte to form the thinner electrolyte layer for coating the surface of the catalyst improves the gas diffusion ability and enables a supply of the reactant (oxygen in the case of FIG. 7) to be fed at a sufficient efficiency to the catalyst. The thinner electrolyte layer, however, results in the insufficient ion-conducting ability and thereby lowers the performance of the catalyst electrode.
As mentioned previously, the electrolyte layer 138 forms the ion transmission path from the electrolyte film towards the gas diffusion layer. The ions passing through the electrolyte layer 138 are gradually fed to the catalyst 134 included in the catalyst electrode 123. The electrolyte layer 138 formed on the surface of the catalyst-carrying carbon granules 136 functions as the transmission path of the ions, which are fed to the catalyst 134 carried on the surface of the catalyst-carrying carbon granules 136, as well as the transmission path of the ions, which are fed to the catalyst carried on the surface of the catalyst-carrying carbon granules that are located closer to the gas diffusion layer. The less amount of the electrolyte to form the thinner electrolyte layer 138 may thus prevent a sufficient amount of ions from moving towards the gas diffusion layer and increase the resistance of the fuel cell.
The prior art technique utilizes the electrolyte for coating the surface of the catalyst to ensure the ion transmission paths, that is, both the microscopic ion transmission path in the vicinity of the catalyst included in the catalyst electrode and the macroscopic ion transmission path from the catalyst electrode to the electrolyte film. As discussed above, however, the prior art technique can not satisfy both the requirements, that is, the improvement in ion-conducting ability and the improvement in gas diffusion ability. Another problem is that a decrease in thickness of the electrolyte layer to ensure the sufficient gas diffusion ability lowers the strength of the whole catalyst layer.